1. Field of the Invention
The invention pertains to the field of biomedical instrumentation. By way of further explanation, this invention pertains to the electronic instrumentation of the cardiovascular condition of a living organism. In still greater particularity the invention provides simultaneous indications of a plurality of cardiovascular conditions. This invention is further characterized by its use of thermodilution techniques to provide an indication of the quantity of blood flow in the cardiovascular system. Additionally this invention can provide an electrocardiogram tracing.
2. Description of the Prior Art
Thermodilution is an application of the calorimetric principle that, in a mixture of fluids at different temperatures, the heat lost by one fluid equals heat gained by the other. For each fluid, the mathematical product of the temperature change, specific heat and mass is equal.
A recognized method for the study of blood circulation involves producing a temperature change in the blood at one point in the blood flow and measuring the temperature change at a second downstream point. Assuming that the measurement of temperature change occurs at a point downstream of the heat source and that the blood's heat content is uniform the measured change will reflect the amount of blood passing through the blood vessel.
In thermodilution studies heat is either removed from or added to the blood stream. One technique involves the injection of a slightly cooler saline solution into the blood. It was introduced by Gegler in 1953 and involved the injection of cold blood or Ringer's solution and measurement of temperature in the pulmonary artery or aorta with thermocouples. The resulting temperature time curve resembled the previously used dye dilution methods of measuring cardiac output. However, this method requires an accurate measurement of the mass and temperature of each injection.
Methods of introducing heat to the blood flow itself have been developed. For example, in U.S. Pat. No. 3,438,253 issued to Fredric W. Kuether et al. on Apr. 15, 1969, a catheter with a heating coil of platinum ribbon, whose resistance changes with temperature, is described. By measuring the energy required to maintain the coil at a constant, elevated temperature, the velocity of blood flow may be determined. While satisfactory for its intended purpose of measuring velocity and direction of blood flow, this device uses continuous heating which could raise the overall temperature of the blood thus reducing accuracy. Furthermore, it is required to measure the cross sectional area of the vessel, which changes during each systole and diastole, and multiply the "velocity" by the cross section of the vessel to obtain volume flow. The velocity of fluid inside the vessels follows a parabolic function (Ruch & Fulten, Medical Physiology & Biphysics p. 248) and therefore the velocity obtained will depend on the position of the catheter inside the vessel and will change with any movement of the catheter to or from the center of the vessel.
In some devices, thermistors, or thermally sensitive resistors composed of an oxidic semiconductive material whose resistance varies with temperature, are employed as temperature measuring devices. A Wheatstone Bridge is used to measure resistance change in the sensing element. The sensing element is the resistance thermometer which is used as one arm of the Wheatstone Bridge. If the other three resistance arm values are known, and the bridge is balanced then no current passes through the galvanometer and the fourth resistance is easily calculated. Once the resistance value of the thermally sensitive resistor is known then the actual temperature is calculated.
Another heating method involves the introduction of heat at one point in the blood flow and the measurement of blood temperature at a downstream point. A device utilizing this method is shown in U.S. Pat. No. 3,359,974 issued to Hassan H. Khalil on Dec 26, 1967. This device uses a standard bilumen or trilumen cardiac catheter tube, about 3 mm dia., with fine lead wires connected to a heater winding, and a distal temperature transducer to measure the temperature change.
The heater winding is 12 to 15 cm of six fine enamel constantan wires, 0.04 mm dia. wound in parallel and soldered to the flattened tip of a lead wire as it emerges from a lumen of the catheter. The coil is heated with high frequency (350 Khz) current in order not to excite the myocardium. The temperature transducer is a fine nickel or platinum resistance thermometer in the form of a bifilar winding over the distal 16 cm of the catheter. The windings are covered with a thin layer of flexible varnish. The temperature transducer is connected to a three-lead thermometer bridge, a D.C. amplifier and recorder.
The catheter is designed so that the heating coil will be in the right atrium and superior vena cava and the temperature transducer will lie in the pulmonary artery. While satisfactory for its intended purposes, long areas of winding are necessary to eliminate errors introduced through incomplete blood mixing and laminar flow. In addition, direct readout is not available with this apparatus and it is therefore necessary to calculate the blood flow from observed data. Also, this device does not provide for electrocardiogram tracing.